High voltage fuses



I 5% oooo oooo ooooo Wiooooooooooooooooo M V/llll"I'II'I'I'AK a h Oct.1, 1957 P. c; JACOBS, JR

HIGH VOLTAGE FUSES Filed April 12, 1956 N Pl L. H i I m M II N INVENTORIPH LIP C. JACOBS JR. Wm M ATT Y United States Patent HIGH VOLTAGE FUSESPhilip C. Jacobs, Jr., Newtonville, Mass., assignor to The Chase-ShawmntCompany, Newburyport, Mass.

Application April 12, 1956, Serial No. 577,855

6 Claims. (Cl. 200-120) This invention relates to fuses, and moreparticularly to current-limiting fuses. Current-limiting fuses blow atthe occurrence of large fault currents in less than a quarter of a cycleof a current wave of 60 C. P. S. after fault inception, i. e. before thefault current can reach the peak intensity which the electric systeminto which the fuse is inserted is capable of supplying. In such fusesarc voltage is being built-up at such a rapid rate that the faultcurrent decays from its limited peak to zero prior to the first naturalcurrent zero of the fault current.

Fuses of the aforementioned type tend to operate unsatisfactorily at theoccurrence of overloads of inadmissible duration, as distinguished frommajor fault currents or short-circuit currents, and this tendency isparticularly noticeable with current-limiting fuses having relativelyhigh voltage ratings, say 3 kv. to 30 kv.

It is, therefore, one object of this invention to providecurrent-limiting fuses which operate satisfactorily both on theoccurrence of fault currents of inadmissible duration, and on theoccurrence of major fault currents or short-circuit currents, and whichfuses lend themselves well to operating voltages in excess of 600 volts,up to many kilovolts.

Another object of the invention is to provide currentlimiting fuseswherein the build-up of arc voltage on the occurrence of overloads ofinadmissible duration is relatively rapid, resulting in a limitation ofarc energy and arcing time, and wherein the voltage surge incident toblowing on occurrence of major fault currents or shortcircuit currentsis relatively moderate, so as not to endanger the insulation of theelectric system into which the fuse is inserted.

Another object of the invention is to provide a currentlirniting fusecomprising means effective upon blowing of the fuse at overload currentsof inadmissible duration for more rapidly increasing the temperature ofthe portions of the fuse link situated immediately adjacent to the pointof initial break and for increasing the contamination of the arc gap toeffect back-burning and growth of the arc gap at an increased rate.

Still another object of the invention is to provide current-limitingfuses having means permitting to apportion at will the amount of axialventing and the amount of radial venting of the arc gap formed uponblowing of the fuse at overloads of inadmissible duration.

A further object of the invention is to provide currentlimiting fuseshaving semi-porous means at the hot spot region thereof tending toinhibit at small current intensities radial venting of the products ofarcing into the pulverulent arc quenching filler but tending to permitsuch venting at high current intensities resulting from major faults.

Other objects of the invention and advantages thereof will, in part, beobvious and in part appear hereinafter.

For a fuller understanding of the invention reference should be had tothe following detailed description and drawing, wherein:

Fig. 1 is in part a longitudinal sectional view taken along 1-1 of Fig.2 and in part a side elevation of a fuse embodying my invention, some ofthe parts thereof being shown as being broken away;

Fig. 2 is in part a longitudinal sectional View taken along 2-2 of Fig.l and in part a front view of the fuse structure shown in Fig. 1;

Fig. 3 is in part a longitudinal sectional view of another fuseembodying my invention taken along 3-3 of Fig. 4 and in part a sideelevation of said fuse; and

Fig. 4 is in part a longitudinal sectional view taken along 44 of Fig. 3and in part a front view of the fuse structure shown in Fig. 3.

Referring now to Figs. 1 and 2, the fuse structure shown thereincomprises a ribbon-type fuse link of a metal having a relatively smallresistivity, a relatively low specific heat and a relatively high fusingpoint. The metal best complying with these requirements is silver.Silver has a resistivity of l.64 1()- ohm-centimeters at 20 deg. C., aresistivity when solid at the melting point of 8.4 10 ohm-centimeters, aresistivity of 16.6 10- ohm-centimeters when liquid at the meltingpoint, a specific heat of 2.60 watt-seo/deg. C.-cm. and a fusing pointof 940 deg. C. As an alternative to silver, though not as desirable assilver, copper may be used in some instances. Copper has a resistivityof 172x10 ohmcentimeters at 20 deg. C,, a resistivity when solid at themelting point of 10.2 l0 ohm-centimeters, a resistivity of 21.3)(10-ohm-centimeters when liquid at the melting point, a specific heat of3.76 watt-sec/deg. C.-cm. and a fusing point of 1063 deg. C. Thecorresponding values for other metals are of quite a different order.The aforementioned constants are conducive to a relatively highcurrent-limiting action, as is well known in the art, and therefore doesnot need to be shown.

Fuse link 6 is arranged in a casing 1 of insulating material, preferablya synthetic-resin-glass-cloth laminate. Both ends of casing 1 are closedby metal washers 3 of which each is provided with a slot for the passageof fuse link 6. The axially outer ends of fuse link 6 are bent in such away that a portion of the link on each end thereof is situated on, andin physical engagement with, the outer surface of casing 1. The portionsof fuse link 6 which are situated on the outer surface of casing 1 arefirmly clamped to it by terminal elements 2 in the form of metal caps.Casing 1 is provided with circular grooves, and the portions of caps 2juxtaposed to these grooves are rolled into these grooves as shown at 4,thus preventing caps 2 from being blown off casing 1 by virtue of thehigh pressures which may occur therein during circuit interruptionsinvolving large amounts of arc energy. The axially outer bent ends oflink 6 are preferably tin coated and soldered to the inner surfaces ofcaps 2. This can be achieved by inductive heating of the fuse aftercomplete assembly thereof. Fuse link 6 comprises an axially innerportion 6A and axially outer portions 6B. The width and cross-sectionalarea of the former portion is smaller than the width and cross-sectionalarea of the latter portions. Shoulders are formed where the axiallyinner fuse link portion 6A meets the axially outer fuse link portions613. The axially inner portion 6A is bent in such a way as to form anangular cavity, which cavity is coextensive with the hot spot of thefuse structure. A body 5 of relatively low fusing point metal isaccommodated in the angular cavity defined by link 6. That metal isadapted to form alloys with the metal-silver or copper-of which fuselink 6 is made which alloys have a relatively high resistivity. Suchalloy-formation or metallurgical reaction takes place at the occurrenceof overloads of inadmissible duration causing fusion of metal body 5.The low fusing point metal body 5 inside the angular cavity formed byfuse link 6 may be tin, or a suitable alloy of tin. Indium or alloys ofindium may likewise be used, as shown in United States Patent 2,703,352to Frederick J. Kozacka, Fuse and Fuse Link of the Time Lag Type, issuedMarch 1, 1955. A mass of a pulverulent arc-quenching medium '7,preferably quartz sand which is chemically reason ably pure, isaccommodated within Casing 1 and the axially outer portions 613 of fuselink 6 are directly submersed in the sand. A sleeve 8 of an inorganicheat resistant insulating material, preferably of woven glass fibers,covers the axially inner portion 6A of link 6. Sleeve 8 covers only theaxially inner portion 6A of link 6 but exposes the axially outerportions 613 of the link to the immediate arc-quenching action of thepulverulent silicious filler 7. The shoulders formed between linkportions 6A and 6B tend to form abutments limiting or precluding axialdisplacement of sleeve 8 along link 6. This tendency is particularlyeffective if the fit of sleeve 8 on link 6 is relatively tight and ifthe weave of sleeve 8 is relatively resilient. The axially outer ends ofsleeve 8 may be closed by plugs 9 of glass wool, as shown. Plugs 9 tendalso to hold sleeve 8 in its position. As an alternative, the ends ofwoven glass fiber sleeve 8 may be closed sufficiently by attaching theinner surfaces of sleeve 8 to the surface of link 6, as by means of asuitable heat resistant adhesive, for instance, water glass. v

The operation of the fuse structure shown in Figs. 1 and 2 is asfollows:

Assuming occurrence of a major fault, then the portions 6A and 6B offuse link 6 will fuse and vaporize in rapid sequence. However, thedifference in time between their respective fusion and vaporizationtends to limit the rate of change of current and, therefore, theresulting surge voltage, below a dangerous level. While sleeve 8operates as a gas barrier, or impediment to the outward flow of gas, aslong as the gas pressure therein is relatively low, if made of anappropriate glass cloth sleeve 8 permits substantial radial venting athigh internal pressures. It may be said that sleeve 8 operates at majorfault currents and concomitant high internal pressures as if it were notpresent and the space occupied by the sleeve 8 were occupied by thesilicious arc quenching medium 7. This statement is correct inasmuch asat high pressures the pores in sleeve 8 permit the radial escape ofproducts of arcing in the same Way as the interstices between the grainsof the quartz filler and inasmuch as the sleeve 8 is being converted, orfused by the heat of the are, into glass-like metal silicates in thesame Way as quartz sand is.

Considering now occurrence of an overload of excessive duration, undersuch circumstances the silver of which link 6 is made will diffuse intothe metal body upon fusion of the latter, forming various brittle alloyswhich have a relatively high resistivity. Metallurgical erosion of link6 results ultimately in the formation of a small break in link 6 andkindling of a low current are. Such arcs have the tendency to hang onfor a long time, to generate large amounts of heat by virtue of theirduration ultimately resulting in thermal destruction and failure of thefuse. This is being precluded in the structure shown by the coaction ofthe vapors resulting from vaporization of the low fusing point metalbody 5 under the heat of the arc and the sleeve 8. These vapors tend tocontaminate the arc gap since their escape is greatly impeded by theaction of sleeve 8. Some of these vapors condense on sleeve 8, but theyare not sufficiently hot to cause fusion of the latter. Suchcondensation tends to further impede venting of the vapors of the lowfusing point metal radially through sleeve 8. These vapors are,therefore, compelled to flow axially along link 6, thus pre-heating thesevered terminals of the link between which are initiation had occurred.This, in turn, is conducive to more rapid growth of the arc gap, morerapid increase of arc voltage and, therefore, more rapid arc extinction.Where the growth of an are gap is relatively slow, the increase of arcvoltage resulting from gap growth and are elongation is again lost, inpart, by virtue of a decay of the arc voltage in the center region ofthe arc, i. e. at the point where the arc had been originally kindled.This decay of arc voltage is due to excessive heat generation at thispoint, resulting from excessive arc duration. Heating of the portions ofthe fuse link immediately adjacent to the arc gap by an abundance of hotare products combined with a thorough contamination of the arc gap by anabundance of metal vapors having a very low ionization potential isequivalent to more rapid back-burning of the fuse link by an arc whosecurrent intensity is relatively higher. In other words, the critical lowcurrent arcs are caused to behave as less critical arcs involvingrelatively higher are currents would behave. This includes a build-up ofarc voltage at a more rapid rate than normally encountered in fuses atlow overload currents.

If the body 5 of low fusing point metal were omitted from the link 6,fusion would ultimately take place at about the same point of the link,this being the hottest spot of the fuse structure. Since the fusingtemperature of silver is considerably higher than that of tin or thelike low fusing point metals, the vapors of silver flowing along thelink might be expected to have an even more intense heating effect uponthe link than those of the low fusing point metal and result in an evenmore rapid back-burning of the arc gap. This is, however, not the case.The difference in behavior, i. e. the greatly accelerated interruptionof the circuit, may be explained by the contamination of the arc gap byan abundance of superheated highly conductive metal vapors, whereassilver vapors have a high resistivity at the vaporization temperature ofsilver.

It is possible to substitute a sleeve of another material for the sleeve8 of woven fiber glass, e. g. a sleeve of a ceramic material. Suchsubstitution entails, however, some disadvantages which may even befatal in some instances. The sleeve of woven fiber glass enables tocontrol the ratio of radial and axial venting, which ratio could not becontrolled if the sleeve were made of a ceramic material whose porosityis insignificant and does not enable radial venting. A sleeve of wovenfiber glass or glass cloth may safely extend along a substantial portionof the entire length of the link without impairing the short-circuitcurrent interrupting ability of the fuse, the sleeve-covered portionstill remaining an effective shortcircuit current interrupter, theglass-cloth sleeve 8 operating in substantially the same fashion as thepulverulent arc-quenching medium 7 itself, and being backed up in depthby the pulverulent arc-quenching medium 7. Any increment of link lengthcovered by a sleeve having not substantially the same properties asglass cloth sleeve 8 would more or less be lost as a means for theinterruption of short-circuit currents and similar major fault currents,i. e. it would be appropriated for interruption of currents of minormagnitude, as overload currents of inadmissible duration.

In Figs. 3 and 4 like parts as in Figs. 1 and 2 have been indicated bythe same reference numerals with a prime added. It is, therefore, notnecessary to describe in detail the structure shown in Figs. 3 and 4inasmuch as it does not differ from that shown in Figs. 1 and 2.

The structure shown in Figs. 3 and 4 comprises a fuse link 6' having aplurality of circular perforations 6" which are equidistantly arrangedalong substantially the entire length of the link. The center portion ofthe link is not perforated and provided with a bent defining an angularcavity receiving a wedge-shaped mass 5' of tin. The tin is applied inhot condition to link 6, and is therefore in intimate contact with thesurface of the link. Sleeve 8' of porous fiber glass cloth covers thetin body 5 and the immediately adjacent portions of link 6. The axiallyouter ends of sleeve or cover 8' are closed, as by threads of fiberglass wound around them.

The operation of the structure shown in Figs. 3 and 4 closely resemblesthat of the structure shown in Figs. 1 and 2. The former is designed forrelatively high currents and relatively low voltages and the latter forrelatively high voltages and relatively low currents. Themultiperforated fuse link structure of Figs. 3 and 4 is not conducive toexcessive surge voltages on blowing of the fuse on major fault currents.

Where higher current intensities are to be carried than those for whichthe structures shown in Figs. 1 to 4 are intended, a plurality of fuselinks may be arranged be tween the terminal elements of the fuse, eachsupporting a body of link-destroying and are gap contaminating lowfusing point metal and each such body being covered by a sleeve,preferably made of resilient porous glass cloth.

It will be understood that it is necessary, for best results, to selectglass cloth woven from glass fibers having a chemical purity comparableto that of the pulverulent arc-quenching filler inside of the casing.The purity of the glass fibers is a matter of particular importancewhere the sleeve extends along a substantial portion of the total lengthof the link.

Having disclosed preferred embodiments of my invention, it is desiredthat the same not be limited to any particular structure disclosed. Itwill be obvious to persons skilled in the art that many modificationsand changes may be made without departing from the broad spirit andscope of my invention. Therefore it is desired that the invention beinterpreted as broadly as possible and that it be limited only asrequired by the prior state of the art.

I claim as my invention:

1. A current-limiting fuse comprising a casing of insulating material,terminal elements on the outer ends of said casing, a ribbon-type fuselink of a metal having a relatively low resistivity and a relatively lowspecific heat conductively interconnecting said terminal elements, abody of a metal on said fuse link having a lower fusing point than thefusing point of the metal of said fuse link adapted to destroy said fuselink by a metallurgical reaction taking place at a temperature below thefusing point of said metal of said fuse link, and a sleeve of heatresistant inorganic insulating material tending to guide the vapors ofsaid lower fusing metal resulting from destruction of said fuse link bysaid metallurgical reaction and consequent arcing in a directionsubstantially longitudinally of said fuse link.

2. A current-limiting fuse comprising a casing of insulating material,terminal elements on the outer ends of said casing, a ribbon-type fuselink of a metal having a relatively small resistivity, a relatively lowspecific heat and a relatively high fusing point conductivelyinterconnecting said terminal elements, a body of a relatively lowfusing point metal on said fuse link adapted to form with the metal ofsaid fuse link alloys having a relatively high resistivity, a mass of apulverulent silicious arcquenching substance inside of said casing inphysical contact with a portion of said fuse link, and a sleeve of aheat resistant inorganic insulating material mounted on said fuse link,covering said body of relatively low fusing point metal and restrainingthe vapors thereof resulting from arcing from freely flowing into saidmass of arcquenching substance.

3. A current-limiting fuse comprising a casing of insulating material,terminal elements on the outer ends of said casing, 21 ribbon-type fuselink of a metal having a relatively low resistivity, a relatively lowspecific heat and a relatively high fusing point conductivelyinterconnecting said terminal elements, a body of a relatively lowfusing point metal on said fuse link adapted to form with the metal ofsaid fuse link alloys having a relatively high resistivity, a mass ofquartz sand inside of said casing surrounding said fuse link, and acover of woven glass fibers mounted on said fuse link at the pointthereof where said body of relatively low fusing point metal is located,the weave of said fibers being sufficiently loose to apportion thevapors of said low fusing point metal generated upon blowing of saidfuse into a transverse flow component across said cover and alongitudinal flow component inside said cover.

4. A current-limiting fuse comprising a casing ofsynthetic-resin-glass-cloth laminate, terminal elements on the outerends of said casing, a multiperforated ribbon-type fuse link of silverconductively interconnecting said terminal elements, a link-destroyingelement comprising tin supported by said fuse link, a mass of quartzsand inside of said casing surrounding said fuse link, and a sleeve ofheat resistant inorganic insulating material covering saidlink-destroying element.

5 A current-limiting fuse as specified in claim 4 wherein said sleeveconsists of glass fiber cloth and is substantially sealed on the axiallyouter ends thereof to impede the out-flow of products of arcing in adirection longitudinally of said sleeve.

6. A current-limiting fuse comprising a casing of insulating material,terminal elements on the outer ends of said casing, a ribbon-type fuselink of a metal having a relatively low resistivity, a relatively lowspecific heat and a relatively high fusing point conductivelyinterconnecting said terminal elements, said fuse link comprising anaxially inner portion having a relatively small width and axially outerportions having a relatively large width and having shoulders formedbetween said inner portion and said outer portions, a body of arelatively low fusing point metal on said fuse link adapted to form withthe metal of said fuse link alloys having a relatively high resistivity,a mass of quartz sand inside of said casing surrounding said fuse link,and a sleeve of woven glass fibers covering said inner portion of saidfuse link but exposing said outer portions thereof to the immediatearc-quenching action of said quartz sand.

References Cited in the file of this patent UNITED STATES PATENTS1,660,828 Bird Feb. 28, 1928 1,861,369 Sundt May 31, 1932 2,532,078Baxter Nov. 28, 1950 2,592,399 Edsall et a1. Apr. 8, 1952 FOREIGNPATENTS 62,851 Norway Aug. 26, 1940 721,236 Germany May 30, 1942

1. A CURRENT-LIMITING FUSE COMPRISING A CASING OF INSULATING MATERIAL,TERMINAL ELEMENTS ON THE OUTER ENDS OF SAID CASING, A RIBBON-TYPE FUSELINK OF A METAL HAVING A RELATIVELY LOW RESISTIVITY AND A RELATIVELY LOWSPECIFIC HEAT CONDUCTIVELY INTERCONNECTING SAID TERMINAL ELEMENTS, ABODY OF A METAL ON SAID FUSE LINK HAVING A LOWER FUSING POINT THAN THEFUSING POINT OF THE METAL OF SAID FUSE LINK ADAPTED TO DESTROY SAID FUSELINK BY A METALLURGICAL REACTION TAKING PLACE AT A TEMPERATURE BELOW THEFUSING POINT OF SAID METAL OF SAID FUSE LINK, AND A SLEEVE OF HEATRESISTANT INORGANIC INSULATING MATERIAL TENDING TO GUIDE THE VAPORS OFSAID LOWER FUSING METAL RESULTING FROM DESTRUCTION OF SAID FUSE LINK BYSAID METALLURGICAL REACTION AND CONSEQUENT ARCING IN A DIRECTIONSUBSTANTIALLY LONGITUDINALLY OF SAID FUSE LINK.